1. Field of the Invention
The present invention relates to a DC-DC converter which transforms DC voltage based on the switching regulator system, and in particular to a switched-mode power supply having a ripple injection function.
2. Description of Related Art
DC-DC converter based on the switching regulator system has been known as a circuit which transforms DC input voltage into a DC voltage of different level. This sort of DC-DC converter includes a driver switching element, a rectifier element and a control circuit. The driver switching element applies DC voltage, received from a DC power supply such as a battery, to an inductor (coil) to thereby feed current thereto and accumulate the energy therein. The rectifier element controls current which flows through the coil, in the time period when energy is discharged while the driver switching element is turned off. The control circuit performs on-off control of the driver switching element.
Conventionally known control methods for the DC-DC converter based on the switching regulator system include voltage control system, current control system and ripple control system. The voltage control system modulates pulse width or frequency of drive pulse for the switching element based on feedback of the output voltage. The current control system is an improved version of the voltage control system. The voltage control system and the current control system, however, suffer from slow response to abrupt changes in load.
On the other hand, the ripple control system performs on-off control of the switching element, upon detecting that the output voltage being monitored falls below (or exceeds) a preliminarily-set threshold value. The ripple control system is intrinsically free from delay ascribable to frequency characteristics of an error amplifier, and may therefore achieve faster response to load as compared to the voltage control system and the current control system.
The DC-DC converter based on the ripple control system generally makes use of a triangular wave (ripples) which appears in the output voltage ascribable to ESR (equivalent series resistance), a resistance component owned by a smoothing capacitor connected to an output terminal. The output voltage is kept constant by monitoring the output voltage using a comparator, and by repetitively turning the switching element on for a predetermined period of time every time the output voltage is detected as being below a predetermined value.
Conventionally, electrolytic capacitors, having a relatively large ESR, have been used as the capacitor for smoothing the output voltage. The ripple control has, therefore, never been interrupted due to shortage of the ripples. However, recent growing need in the market of digital home electric appliances is directed to ceramic capacitors having smaller ESR, aiming at reducing the ripples per se, the external dimensions, and the cost and at improving the reliability. Capacitors having small ESR, however, scarcely produce the ripple component, and disable the ripple control. One known proposal is directed to a switched-mode power supply configured to inject the ripple component into feedback voltage of the output voltage (see Japanese Examined Patent No. 4610588).
Further, an invention relating to a switched-mode power supply including an integrating circuit which generates ripple voltage similar to ripple current that feeds into an inductor, the integrating circuit being connected in parallel with the inductor is suggested (see Japanese Examined Patent No. 4613986). In this invention, the generated ripple voltage is transformed into current to be supplied, as a working current, to a comparator which compares the feedback voltage and the reference voltage and to be supplied, as an input voltage, to the comparator.